Controlling the pH, conductivity and/or the level of target ions of a liquid composition is an important industrial process used within such broad technical fields as metal refining and purification of organic substances from a fermented liquid.
A number of processes have been introduced in order to control the level of target ions or pH of a liquid composition. Among said processes are micro- and ultrafiltration processes, ion-exchange processes and electrodialysis processes.
E.g. bioreactors are used extensively in the industry for production of a wide range of organic chemicals, pharmaceutical proteins, amino acids, starter cultures, biofuels etc., or for biodegradation purposes. Most often, the bioreactor contains microorganisms that require a certain pH level to sustain optimal functionality.
Standard means for pH regulation in bioreactors includes titration of alkaline or acidic neutraliser directly into the bioreactor, to neutralize acidic or alkaline metabolites produced by the microorganisms. However, salts of these metabolites often inhibit growth when reaching a certain concentration.
The build-up of some salts and metabolites, such as inhibitors for biological reactions, sets a limit to the productivity of a bioreactor in normal batch operations. Possible solutions to minimize inhibition include perfusion systems, where fermentation broth with inhibitors is continuously extracted through a filter process (e.g. ultra- or microfiltration), while retaining the microorganisms and adding fresh substrate solution to the fermenter. In perfusion systems, it is still necessary to regulate bioreactor pH through titration of neutralizer and valuable substrate components are lost with the permeate.
The use of ion-exchange membranes, as utilized in electrodialysis and the so-called Donnan Dialysis processes, allows for a more selective extraction of small charged species compared to membranes typically used in ultra- and microfiltration. However, conventional electrodialysis suffers from membrane fouling when combined directly with bioreactors and neutralizer titration is still necessary for pH control.
EP patent 1 237 823 discloses an apparatus and a method for transferring ionic species from a first liquid to a second liquid in an electro-enhanced dialysis cell comprising either cation exchange membranes or anion exchange membranes.
U.S. Pat. No. 5,114,554 discloses a process for removing acid from a cathodic electrocoating bath in which electroconductive substrates are being coated with cationic resins, at least a portion of the coating bath being subjected to an ultrafiltration, and at least a portion of the ultrafiltrate being subjected to a specific electrodialysis treatment in a direct current operated electrodialysis cell comprising anion exchange membranes.